1. Field of the Invention
The present invention relates to a radiation imaging apparatus and a radiation imaging system suitably applicable to medical diagnosis and industrial non-destructive inspection. In the description of the specification, a radiation includes electromagnetic waves such as X- and gamma rays and alpha and beta rays.
2. Description of the Related Art
Nowadays, X-ray imaging in a hospital has been changing from a conventional analog system using films to a digital system. Digitized X-ray imaging easily solves problems which have been pointed out so far such as the safekeeping of films, the management of developers, radiographing time period and burden to patients at the time of failure in radiographing and enables providing a new medial environment to meet the needs of the times.
A CR (computed radiography) system using photostimulable phosphor called an imaging plate (IP) as a digital X-ray imaging system has dominated since 1980s and has had a share in digitization. The CR system has indeed an aspect of digitization, but it requires two-step process in which a latent X-ray image in the IP by X-ray imaging is scanned by laser beams to obtain images. For this reason, the CR system has still a problem with work flow in that time is needed from radiographing to acquiring images.
A digital X-ray imaging apparatus provided with radiographing detecting elements containing as a main ingredient amorphous silicon and amorphous selenium have been practically used in recent years. The former is of indirect type in which X-ray images are converted into visible images by a phosphor containing as a main ingredient CsI:TI or Gd2O2S:Tb and the visible images are converted into electric signals by X-ray detecting elements containing as a main ingredient amorphous silicon. On the other hand, the latter is of direct type in which X-rays are directly converted into electric signals by X-ray detecting elements containing as a main ingredient amorphous selenium. Both are capable of realizing a wider and thin X-ray imaging apparatus, so that it is also referred to as “flat panel detector (FPD)” and characterized in that the time required from imaging to observing images is very short. In a recent digital system, a demand for the CR system is still active, but a demand for the FPD system is gradually developing.
In the next place, a moving image radiographing (fluoroscopic radiography) is briefly described below. In fluoroscopic radiography for gastric as an example of the moving image radiographing, the inner wall of a stomach or duodenum with barium swallowed as a contrast medium is observed by an imaging apparatus called an image intensifier (II). The II is very sensitive and a device widely used for moving image radiographing. The II converts an X-ray image into a visual image and then converges the visual image using an electronic lens, which offers a drawback in that an apparatus becomes bulky and heavy and peripheral images are greatly distorted. In addition, it is pointed out that the II is so small in dynamic range that a problem with halation is caused. Furthermore, the II remarkably deteriorates in its characteristics and is short in lifetime, so that it needs to be replaced every three to five years depending on frequency in use. In a fluoroscopic radiographing for gastric, when a still image is photographed by fluoroscoping by the II with a film loaded.
The II is also used for fluoroscopic radiographing of heart or brain as well as for a fluoroscopic inspection of gastric. Since the moving image radiographing exposes a patient to X rays for a long time, it is necessary to reduce the dose of X rays per unit time in radiographing. For this reason, an X-ray imaging apparatus needs to be higher in sensitivity than that used for the still image radiographing.
An FPD capable of both photographing a still image and radiographing a moving image has been proposed in recent years. Radiographing a moving image needs to ensure a high frame rate unlike photographing a still image. In general, cardioangiography requires a frame rate of 30 FPS depending on the part and the purpose of radiographing, so that S/N is improved by using, for example, a pixel binding method to further increase the frame rate.
The FPD can generate such turbulence in a signal that a certain noise quantity is superimposed on a line basis. This is referred to as “line noise” and brings about a horizontal (in the direction of a line) linear artifact illustrated in FIG. 23 to significantly decrease image quality.
The line noise is most probably attributed to the following reason; a generated noise gets into a driving signal output from a driving circuit because switching elements are collectively operated on a line basis or into a signal wiring for some reason, and thereafter is transferred at the same time. The line noise is liable to be generated also at the time of resetting the capacitors of the signal wiring and the capacitive elements in a reading circuit because the resetting is performed on a line basis. The line noise can get into a signal from the driving circuit and various power supplies (including GND) or it generates in an adjacent appliance and gets into a signal through space. The line noise getting into at the time of establishing a proper electric potential, for example, immediately before the transfer of signal electric charges is finished or resetting is finished is turned into a line noise on a line basis.
In general, a random noise is known as one of noises resulting from the graininess of an image. The noise is generated by a shot noise resulting from the dark current of a sensor (a radiation detecting element), a thermal noise of a switching element, a thermal noise generated in the resistance of a drive wiring or a signal wiring and thermal noise from the operational amplifier in a reading circuit. The line noise significantly degrades image quality if the image is formed on a line basis as illustrated in FIG. 23. If the line noise is generated not singularly, as illustrated in FIG. 23, but randomly also on a line basis, the relationship between the standard deviation σ(R) of the random noise and the standard deviation σ(L) of the line noise can be experientially σ(L)=σ(R)· 1/10 or less. That is to say, the line noise is extremely conspicuous on an image and is very difficult to reduce. Particularly in the moving image radiographing, a little dose of X rays produces a problem in that the line noise is liable to be conspicuous. For example, the following is cited as document on conventional art related to line noise on an imaging apparatus. Japanese Patent Application Laid-Open No. 2004-007551 discloses an imaging apparatus which is provided with a line noise detecting unit for detecting the existence of a line noise in the imaging output of a two-dimensional area sensor stored in a memory circuit and calculates the output quantity of the line noise to remove the line noise from the imaging output. Furthermore, U.S. Pat. No. 6,734,414 discloses an imaging apparatus in which drive wirings are not connected to pixels on a line basis, but randomly connected to prevent a horizontal linear line noise from being generated as illustrated in FIG. 20.